Most of the current cathode materials in lithium-ion batteries exhibit a specific capacity significantly lower than 200 mAh/g (e.g., 140 mAh/g for LiCoO2). One exception is vanadium-based materials (e.g. VO2, LixVO2, V2O5, LixV2O5, V3O8, LixV3O8, LixV3O7, V4O9, LixV4O9, V6O13, LixV6O13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5) that exhibit exceptional specific capacity due to their ability to incorporate more than one lithium ion per vanadium atom. For example, a specific discharge capacity of 442 mAh/g [Ref.1] was obtained when three lithium ions intercalate into V2O5. Nanometer thick LixV2O5 nano-belts with the δ-type crystal structure, synthesized by a hydrothermal treatment of Li+-exchanged V2O5 gel, were found to exhibit a specific capacity of 490 mAh/g [Ref.2]. Lithium trivanadate (LiV3O8) is another vanadium oxide that also can accommodate several lithium ions per formula [Ref.3-5].
However, vanadium oxide-based materials have not been used to any significant extent in battery industry likely due to the following reasons:                (a) Electrochemical properties (e.g. specific capacity, capacity retention, and rate capability) are highly sensitive to vanadium oxide synthesis and electrode fabrication conditions. A high specific capacity, high capacity retention upon repeated charges/discharges, and/or high rate capability (ability to possess a high capacity at a high charge/discharge rate) have not been consistently achieved.        (b) In those cases of a high first-cycle capacity (e.g. >300 mAh/g), the capacity typically decays very rapidly upon repeated charges/discharges. Reasonable capacity retention could only be achieved with those compositions that exhibit an initial specific capacity <<270 mAh/g (however, this value is lower than the highest reported value by 45%). As demonstrated in Table 1 below, it has been difficult to find a vanadium oxide composition that exhibits a capacity >270 mAh/g and good capacity retention (e.g. >90% of the first-cycle capacity after 100 cycles) in a half-cell configuration (i.e., using lithium foil as a counter electrode). A pressing need exists for a vanadium oxide-based material with both high discharge capacity and good capacity retention beyond 100 cycles.        (c) The specific capacity of this class of cathode materials is highly sensitive to the charge/discharge rate, exhibiting a dramatic decrease in capacity with just a minor or moderate rate increase. Even with a nano-structure (nano-particle, nano-rod, nano-wire, nano-sheet, or nano-belt), these vanadium-based cathodes still fall short in terms of providing a good capacity at a high rate (i.e. they exhibit poor rate capability).        
TABLE 1Specific capacity and capacity retention of various vanadium oxide-based cathodematerials as a function of current density (rate) and cycle number.CurrentCapacitySynthesis MethoddensityCapacity, mAh/gretention(Ref. No.)Composition(mA/g)(cycle number)(%)Hydrothermal (6)LiV3O8120212.8 (1) → 152 (18) 72Sol-gel (7)Li1.2V3O860281 (2) → 200 (40)71Sol-gel (8)Li0.96Ag0.04V3O8150 328 (1) → 252.7 (50)77Flame pyrolysis (9)LiV3O8 nanoparticles100271 (1) → 180 (90)66Solution process (10)LiV3O8-PP composite40184 (1) → 183 (50)99Spray drying (11)Li1.1V3O8116260 (2) → 220 (60)84Hydrothermal (12)LiV3O8 nano-rod100 247 (1) → 236 (100)96Surfactant-assistedLiV3O8 nano-rod120182 (2) → 180 (60)99polymer precursor (13)Soft chemistry (5)LiV3O8 nano-rod100 320 (2) → 250 (100)78Chelating and PEG-LiV3O8 nano-sheets100 260 (2) → 262 (100)100modifying (1)prepared at different100 194 (2) → 181 (100)93temperatures100 166 (2) → 157 (100)94Hydrothermal (2)LixV2O5 nano-belts, >5-1520490 (1) → 405 (50)83nm thick100400 (5) → 370 (25)92Solid state reactionLiV3O8 nano-belts, 2020 356 (1) → 298.6 (30)84(14)run thick50 274 (1) → 234.5 (30)85100 234 (1) → 195.5 (30)84.5Hydrothermal (15)LixV3O7•H2O (x =20409 (1) → 260 (20)644.32); nano belts, 30100340 (1) → 240 (20)71nm thick1000270 (1) → 170 (20)6310000100 (1) → 50 (20) 50Electro-spinning andLixV2O5 nano-belts,0.1 A/cm2350 (1) → 240 (20)69hydrothermal (24)10-20 nm thickHydrothermal (25)V3O7•H2O, nano belts,30 253 (1) → 228.6 (50)9020 nm thickList of References Cited:(1) Anqiang Pan, Ji-Guang Zhang, Guozhong Cao, Shuquan Liang, Chongmin Wang, Zimin Nie, Bruce W. Arey, Wu Xu, Dawei Liu, c Jie Xiao, Guosheng Li and Jun Liu, “Nanosheet-structured LiV3O8 with high capacity and excellent stability for high energy lithium batteries,” J. Materials Chem., DOI: 10.1039/c1jm10976f.(2) Dmitrii A. Semenenko, Daniil M. Itkis, Ekaterina A. Pomerantseva, Eugene A. Goodilin, Tatiana L. Kulova, Alexander M. Skundin, Yurii D. Tretyakov, “LixV2O5 nanobelts for high capacity lithium-ion battery cathodes,” Electrochemistry Communications, 12 (2010) 1154-1157.(3) Q. Shi, R. Z. Hu, L. Z. Ouyang, M. Q. Zeng and M. Zhu, High-Capacity LiV3O8 Thin-Film Cathode with a Mixed Amorphous-Nanocrystalline Microstructure Prepared by RF Magnetron Sputtering, Electrochem. Commun., 2009, 11, 2169-2172.(4) Y. Feng, F. Hou and Y. Li, A New Low-temperature Synthesis and Electrochemical Properties of LiV3O8 Hydrate as Cathode Material for Lithium-Ion Batteries, J. Power Sources, 2009, 192, 708-713.(5) A. Pan, J. Liu, J.-G. Zhang, G. Cao, W. Xu, Z. Nie, X. Jie, D. Choi, B. W. Arey, C. Wang and S. Liang, Template Free Synthesis of LiV3O8 Nanorods as a Cathode Material for High-Rate Secondary Lithium Batteries, J. Mater. Chem., 2011, 21, 1153-1161.(6) J. Xu, H. Zhang, T. Zhang, Q. Pan and Y. Gui, Influence of Heat-Treatment Temperature on Crystal Structure, Morphology and Electrochemical Properties of LiV3O8 Prepared by Hydrothermal Reaction, J. Alloys Compd., 2007, 467, 327-331.(7) J. G. Xie, J. X. Li, H. Zhan and Y. H. Zhou, Low-temperature Sol-gel Synthesis of Li1.2V3O8 from V2O5 Gel, Mater. Lett., 2003, 57, 2682-2687.(8) J. Sun, L. Jiao, H. Yuan, L. Liu, X. Wei, Y. Miao, L. Yang and Y. Wang, Preparation and Electrochemical Performance of AgxLi1−xV3O8, J. Alloys Compd., 2009, 472, 363-366.(9) S.-H. Ng, T. J. Patey, R. Buchel, F. Krumeich, J.-Z. Wang, H. Liu, S. E. Pratsinis and P. Novak, Flame Spray-pyrolyzed Vanadium Oxide Nanoparticles for Lithium Battery Cathodes, Phys. Chem. Chem. Phys., 2009, 11, 3748-3755.(10) S. Y. Chew, C. Feng, S. H. Ng, J. Wang, Z. Guo and H. Liu, Low-Temperature Synthesis of Polypyrrole-Coated LiV3O8 Composite with Enhanced Electrochemical Properties, J. Electrochem. Soc., 2007, 154, A633-A637.(11) N. Tran, K. G. Bramnik, H. Hibst, J. Prolss, N. Mronga, M. Holzapfel, W. Scheifele and P. Novak, Spray-drying Synthesis and Electrochemical Performance of Lithium Vanadates as Positive Electrode Materials for Lithium Batteries, J. Electrochem. Soc., 2008, 155, A384-A389.(12) H. Liu, Y. Wang, K. X. Wang, Y. R. Wang and H. S. Zhou, Synthesis and Electrochemical Properties of Single-Crystalline LiV3O8 Nanorods as Cathode Materials for Rechargeable Lithium Batteries, J. Power Sources, 2009, 192, 668-673.(13) A. Sakunthala, M. V. Reddy, S. Selvasekarapandian, B. V. R. Chowdari and P. C. Selvin, Preparation, Characterization, and Electrochemical Performance of Lithium Trivanadate Rods by a Surfactant-Assisted Polymer Precursor Method for Lithium Batteries, J. Phys. Chem. C, 2010, 114, 8099-8107.(14) Weizhong Wu, Jie Ding, Hongrui Peng, Guicun Li, “Synthesis and electrochemical properties of single-crystalline LiV3O8 nanobelts for rechargeable lithium batteries,” Materials Letters, 65 (2011) 2155-2157.(15) Shaokang Gao, Zhanjun Chen, Mingdeng Wei, Kemei Wei, Haoshen Zhou, “Single crystal nanobelts of V3O7•H2O: A lithium intercalation host with a large capacity,” Electrochimica Acta, 54 (2009) 1115-1118.(16) Aruna Zhamu and Bor Z. Jang, “Hybrid Nano Filament Cathode Compositions for Lithium Ion and Lithium Metal Batteries,” U.S. patent application No. 12/009,259 (Jan. 18, 2008).(17) Jinjun Shi, Aruna Zhamu and Bor Z. Jang, “Conductive Nanocomposite-based Electrodes for Lithium Batteries,” U.S. patent application No. 12/156,644 (Jun. 04, 2008).(18) Aruna Zhamu, Bor Z. Jang, and Jinjun Shi, “Nano Graphene Reinforced Nanocomposite for Lithium Battery Electrodes,” U.S. patent application No. 12/315,555(Dec. 04, 2008).(19) Aruna Zhamu, Bor Z. Jang, and Jinjun Shi, “Process for Producing Nano Graphene Reinforced Nanocomposite for Lithium Battery Electrodes,” U.S. patent application No. 12/319,812 (Jan. 13, 2009).(20) A. Zhamu and Bor Z. Jang, “Conductive Graphene Polymer Binder for Electrochemical Cell Electrodes,” U.S. patent application No. 12/655,172 (Dec. 24, 2009).(21) Aruna Zhamu, Jinjun Shi, Guorong Chen, M. C. Wang, and Bor Z. Jang, “Graphene-Enhanced Cathode Particulates for Lithium Batteries,” U.S. patent application No. 12/807,471 (Sep. 07, 2010).(22) Y. Ding, et al. “Preparation of nano-structured LiFePO4/graphene composites by co-precipitation method,” Electrochemistry Communications, 12 (2010) 10-13.(23) Xufeng Zhou, Feng Wang, Yimei Zhu, and Zhaoping Liu, “Graphene modified LiFePO4 cathode materials for high power lithium ion batteries,” J. Mater. Chem., 2011, 21, 3353.(24) Chunmei Ban, Natalya A. Chernova, M. Stanley Whittingham, “Electrospun nano-vanadium pentoxide cathode,” Electrochemistry Communications 11 (2009) 522-525.(25) Hui Qiao, Xianjun Zhu, Zhi Zheng, Li Liu, Lizhi Zhang, “Synthesis of V3O7 H2O nanobelts as cathode materials for lithium-ion batteries,” Electrochemistry Communications 8 (2006) 21-26.
Further, due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 5%-15%, must be added into the electrode. However, the conductive additive is not an electrode active material. The use of a non-active material means that the relative proportion of an electrode active material, such as vanadium oxide, is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 10% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active material is only 85%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-170 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.
State-of-the-art carbon black (CB) materials, as a conductive additive, have several drawbacks: (a) CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature (largest dimension-to-smallest dimension ratio or aspect ratio ˜1) and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix (e.g. lithium cobalt oxide and lithium iron phosphate), a large amount of CBs is required to reach a percolation threshold where the CB particles are combined to form a 3-D network of electron-conducting paths. (b) CBs themselves have a relatively low electrical conductivity and, hence, the resulting electrode remains to be of relatively low conductivity even when the percolation threshold is reached. A relatively high proportion of CBs (far beyond the percolation threshold) must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.
Clearly, an urgent need exists for a more effective electrically conductive additive material. Preferably, this electrically conductive additive is also of high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
There have been several attempts to use other carbon nano-materials than carbon black (CB) or acetylene black (AB) as a conductive additive for the cathode of a lithium battery. These include carbon nano-tubes (CNTs), vapor-grown carbon nano-fibers (VG-CNFs), and simple carbon coating on the surface of cathode active material particles. The result has not been satisfactory and hence, as of today, carbon black and artificial graphite particles are practically the only two types of cathode conductive additives widely used in lithium ion battery industry. The reasons are beyond just the obvious high costs of both CNTs and VG-CNFs. The difficulty in disentangling CNTs and VG-CNFs and uniformly dispersing them in a liquid or solid medium has been an impediment to the more widespread utilization of these expensive materials as a conductive additive. For the less expensive carbon coating, being considered for use in lithium iron phosphate, the conductivity of the carbon coating (typically obtained by converting a precursor such as sugar or resin via pyrolyzation) is relatively low. It would take a graphitization treatment to render the carbon coating more conductive, but this treatment requires a temperature higher than 2,000° C., which would degrade the underlying cathode active material.
Instead of following the CNT, CNF, or carbon coating approaches, our research groups have invented several different approaches [Ref. 16-21] that make use of nano graphene platelets (NGPs) as a critical ingredient in a cathode. The NGP in these earlier reports refers to single-layer or multi-layer graphene or graphene oxide sheets (having an oxygen content of 0-46% by weight). These earlier approaches developed by us include:                1) Using NGP as a substrate to support a cathode active material coating [Ref. 16];        2) Using NGP as a 3-D network of graphene-based conducting pathways with cathode active particles bonded thereto via a conductive binder [Ref. 17];        3) Using NGP as a reinforcing agent for a matrix (e.g. carbon matrix) with cathode active material particles dispersed in graphene sheet-reinforced carbon matrix [Ref 18 and 19];        4) Using NGP as a graphene-based conductive binder polymer (to replace non-conductive polymer, such as PVDF and PTFE) due to graphene's surprisingly high adhesive power [Ref. 20]; and        5) Using NGP as an encasing agent that embraces an aggregate of cathode active material particles to form graphene nano-nodules for improved cathode conductivity [Ref 21].        
Since the discovery of the graphene-based approaches by our research group, there has been increasing interest in following these approaches. For instance, Ding, et al investigated the electrochemical behavior of LiFePO4/graphene composites [Ref 22], which is essentially a variant of our earlier work [Ref 16]. Zhou, et al [Ref 23] also made use of the concept of 3-D graphene network as proposed by us in [Ref. 17].
However, no prior work has been reported on using graphene, graphene oxide, or graphene fluoride sheets to modify, regulate, control, or tailor-make the structure, morphology, shapes, and sizes of various vanadium oxide compositions to achieve a high specific capacity, stable cycling behavior (high capacity retention percentage), and/or high rate capability (high capacity at high charge/discharge rates, e.g., >3 C or even >20 C rate). In battery industry, 1 C means completing the charge or discharge procedure in 1 hour and n C means completing the charge or discharge procedure in (1/n) hours (20 C means completing the procedure in 1/20 hours or 3 minutes). No prior art has taught about the graphene-vanadium oxide composites prepared by precipitating vanadium oxide-type cathode material in the presence of graphene, graphene oxide, or graphene fluoride sheets. These sheets are herein surprisingly found to have totally altered the structure, morphology, shapes, and dimensions of the vanadium oxide nano-structure and, hence, have dramatically improved the electrochemical properties of vanadium oxide, reaching unprecedented levels of performance in terms of first-cycle capacity, capacity retention (cycle stability), and/or high-rate capability.
Thus, the objects of the present invention are to:                (A) Provide a vanadium oxide-based cathode active material that consistently exhibits a combination of excellent electrochemical properties (e.g. a combination of good specific capacity, capacity retention, and rate capability, not just one type of good property).        (B) Provide a vanadium oxide-based cathode active material that exhibits a high first-cycle capacity (e.g. >300 mAh/g) and an ability to maintain a high capacity for a long cycle life (e.g. >90% retention after >100 charge/discharge cycles or >80% retention after >200 cycles). By 90% retention we mean maintaining 90% of the original specific capacity after a specified number of cycles.        (C) Provide a vanadium oxide-based cathode active material that exhibits a high specific capacity even at a high C rate (e.g., >400 mAh/g at 1 C, >350 mAh/h at 2.5 C, and/or >300 mAh/g at >10 C). These high specific capacities at such high charge/discharge rates have not been achieved in the lithium-ion battery industry.        (D) Provide a vanadium oxide-based cathode active material that exhibits a nano-structure (nano-particle, nano-rod, nano-wire, nano-sheet, or nano-belt) having a dimension (e.g. average thickness) smaller than 10 nm, preferably smaller than 5 nm. Although Semenenko et al [Ref 2] reported LixV2O5 nano-belts having a thickness range of 5-15 nm, the average thickness is exactly equal to 10 nm (not <10 nm).        (E) Provide a vanadium oxide-based composite material that is more thermally and electrically conductive than the corresponding vanadium oxide alone.        (F) Provide a vanadium oxide-based composite material composed of multiple primary particles of vanadium oxide that, in combination with graphene, graphene oxide, or graphene fluoride sheets, and an optional carbon material, are aggregated into secondary particles (herein referred to as particulates) that are more amenable to the production of cathodes having a good tap density using current production equipment. This is significant since it is very difficult to fabricate electrodes directly from primary nano-scaled particles of electrode-active materials. Also, non-spherical particles normally lead to electrodes with a low tap density (electrode weight/electrode volume). In other words, the tap density of the presently invented composite material is higher than that of vanadium oxide particles when used alone (without graphene, graphene oxide, or graphene fluoride) in an electrode.        (G) Provide a conductive additive or modifier that is capable of helping multiple primary particles of a vanadium oxide material aggregate into secondary particles that contain a 3-D network of electron-conducting paths, imparting exceptional conductivity to the cathode, and enabling the cathode to become high-rate capable.        (H) Provide a vanadium oxide-based composite material wherein graphene, graphene oxide, or graphene fluoride sheets serve as heterogeneous nucleation sites to encourage nucleation of crystals from a large number of sites (i.e. to promote nucleation as opposed to growth of vanadium oxide crystals), resulting in the formation of vanadium oxide crystals in the form of nanoparticle, nano-rod, nano-wire, nano-belt, or nano-sheet having a size (e.g. average thickness) smaller than 30 nm, preferably smaller than 20 nm, further preferably smaller than 10 nm, and most preferably smaller than 5 nm.        